Titanium powder sintered compact

ABSTRACT

Provided are a porous sintered compact suitable for a filter, a power feeder in a polymer electrolyte membrane type water electrolyzer, a current collector in a solid polymer fuel cell and in addition a liquid dispersion plate, especially an ink dispersion plate for an ink jet printer ink and the like. A titanium powder sintered compact made of a plate-like porous compact is obtained by sintering spherical powder made of titanium or a titanium alloy produced by means of a gas atomization method. A void ratio in the range of from 35 to 55% is realized by filling without applying a pressure and sintering without applying a pressure.

TECHNICAL FIELD

The present invention relates to a porous titanium powder sinteredcompact employed as a filter, a power feeder in a polymer electrolytemembrane type water-electrolyzer, a current collector in a solid polymerfuel cell and in addition a liquid dispersion plate, especially an inkdispersion plate for an ink jet printer and the like.

BACKGROUND ART

A metal powder sintered compact has been adopted as one of filtersemployed in the chemical industry, the polymer industry, the chemicalsindustry and others. As metals herein, there have been used generallybrass, stainless steel and, recently, titanium.

Titanium is greatly excellent in corrosion resistance, acid proofnessand the like as compared with stainless steel, but on the other hand,extremely poor in moldability. Hence, generally, a titanium sinteredfilter has been fabricated according to a method in whichhydrogenation/dehydrogenation titanium powder accepted as having acomparatively good moldability is molded with a die press, followed bysintering, and furthermore, a method disclosed in JP A 1995 (H7)-238302uses titanium sponge powder comparatively good in moldability similarlyto hydrogenation/dehydrogenation powder.

Such titanium sintered filters have started finding applications thereofto, for example, highly corrosion resistant filters for a carrier gasinlet section of a gas chromatography apparatus, production of food suchas a liquid condiment, and a liquid pigment.

Filters well used in various application fields have been faced a demandfor maximum pore diameters adapted for respective purposes of usage. Theterm, “a maximum pore diameter” is used as an index expressing the sizeof a particle removable by a filter, wherein with the same value of amaximum pore diameter, it may be considered that filters havingrespective pore shapes different from each other can remove particleshaving at least the same diameter. A filter with the smaller pressuredrop is more requested among filters with the same maximum porediameter. For example, as the carrier gas inlet section filter for a gaschromatography apparatus, a filter has been desired that is not onlyexcellent in corrosion resistance but also especially has a maximum porediameter of 70 μm or less and a smaller pressure drop.

In a titanium sintered filter using hydrogenation/dehydrogenation powderor titanium sponge powder, however, there exists a constraint to disablea pressure drop to be reduced to a small value in a case where a maximumpore diameter is adjusted to 70 μm or less.

A titanium sintered filter using hydrogenation/dehydrogenation powder ortitanium sponge powder has another problem that the filter is very hardand fragile without flexibility; therefore it is easy to be broken ifbeing thin and difficult to fabricate the filter large in area. Inaddition, since bending is difficult at room temperature, a productcannot be fabricated by bending, causing a problem of a high fabricationcost except for a plate-like shape.

For example, a case arises where there is requested a titanium sinteredfilter of the shape of a cylinder, of the order of 40 mm in diameter(with a radius of curvature of 20 mm), whereas since it is impossible tobend a titanium sintered compact in the form of a flat plate into theform of a cylinder at room temperature, a necessity arises for workingby cold isostatic pressing called CIP for short as described in JP No.2791737, thereby increasing in fabrication cost cannot be avoided.

Even with hydrogenation/dehydrogenation titanium powder and titaniumsponge powder, moldability is inferior to that of stainless steel.Hence, it is difficult to mold the titanium powders into shapes exceptfor a thin flat plate. Therefore, it is also difficult to directly molda filter in the shape of a cylinder without resorting to bendingprocess.

That is, in a case where a sintered compact in the shape of a cylinderis fabricated by press molding using hydrogenation/dehydrogenationtitanium powder or titanium sponge powder, a press force in a directionof height does not effectively act, which results in difficulty inmolding a middle portion in the direction of height; therefore acylinder with a large height cannot be produced though a low-profilering can be produced. While a cylinder with a large height can befabricated by cold isostatic pressing called CIP instead of a press, ahigh cost is encountered, making CIP improper as a fabrication methodfor a sintered filter. Therefore, while it is imagined to stack ringsalong the central axis direction to weld the rings, needless to say thatthus fabricated sintered filter is much higher in cost as compared witha sintered filter fabricated by press molding stainless steel powder.

Incidentally, a method is described in JP No. 2791737 in which stainlesssteel powder is subjected to cold isostatic pressing to fabricate asintered filter in the shape of a cylinder.

As a different problem of a titanium sintered filter usinghydrogenation/dehydrogenation titanium powder or titanium sponge powder,low reverse-washing property arises. That is, sizes and shapes ofcavities are randomly distributed in titanium sintered filters made ofeach powder. While a filter of this kind is used over a long termrepeating reverse-washing, if sizes and shapes of cavities are random,solid matter trapped therein are not sufficiently removed even withreverse-washing. Hence, the problem of low reverse-washingreproducibility has remained.

It is described in JP No. 2791737 that, as for a stainless steel filter,a diameter of a cavity is increased in a direction from the frontsurface to the rear surface of a stainless steel sintered filter inorder to enhance reverse-washing reproducibility of a metal powdersintered filter. To be concrete, a slurry obtained by dispersing finepowder into a binder resin solution is applied on the surface of aporous compact obtained by presintering and thereafter, sintering of theporous compact is conducted, thereby forming a skin layer with finepores on the surface of the porous compact.

In such a multilayer structure, since almost all solid matter in atreated liquid is trapped in the skin layer with fine pores formedtherein and no foreign matter is trapped in cavities inside the baselayer, the solid matter trapped and accumulated in the skin layer iseasy to be removed by reverse-washing. On the other hand, the followingproblem arises however.

Stainless steel is inferior to titanium in corrosion resistance.Furthermore, stainless steel powder used here is composed of irregularlyshaped particles produced by a water atomization technique; therefore,sizes and shapes of cavities in a sintered compact are random in notonly the base layer but also the skin layer. Besides, since the skinlayer does not receive the action of press molding though the base layerreceives the action of press molding, sizes and shapes of fine pores inthe skin layer are more random than in the base layer. For this reason,solid matter remains in the skin layer after reverse-washing, therebydisabling reverse-washing reproducibility as high as expectation. Inaddition, since a void ratio of the skin layer receiving no pressmolding is largely different from that of the base layer receiving pressmolding, there also arises a risk that permeability of a treated liquidis lowered.

A sintering compact is also used as a power feeder in a waterelectrolytic cell producing hydrogen and oxygen using a polymerelectrolytic film. Concrete description will be given of the waterelectrolytic cell; a construction is generally employed in which a unitis formed by placing power feeders on both surfaces of a film electrodelaminates formed by laminating catalyst layers onto both surfaces of thepolymer electrolyte film, multiple of units are stacked and electrodesare provided on both sides thereof.

The power feeders herein are made each of a porous conductive plate andplaced in close contact with an adjacent film electrode laminates. Why aporous conductive plate is used as a power feeder is that a current isrequired to flow through, that water is required to be supplied for awater electrolytic reaction and that gas generated in the waterelectrolytic reaction is quickly expelled out.

A structure of a fuel cell using a polymer electrolytic film is also thesame as that of the water electrolyzer and porous conductive plates areplaced on both surfaces of a film electrode laminates. In the case of afuel cell, since an electric power is obtained with hydrogen as fuel,the porous conductive plates are called current collectors.

As to a porous conductive plate such as a power feeder in such a polymerelectrolyte membrane type water electrolyzer or a current collector insuch a solid polymer fuel cell, titanium have been studied because of anecessity for characteristics enabling a material to be used in anoxidizing atmosphere, and among titanium with actual natures andconditions, especially a sintered compact has drawn attention since asurface is smooth, it is difficult to damage an adjacent film electrodelaminates and a proper void ratio can be attained with ease.

As porous conductive plates made of a titanium sintered compact, thereare exemplified: a titanium powder sintered plate obtained by sinteringpowder obtained by crushing titanium sponge or powder produced bypulverizing titanium sponge by hydrogenation and dehydrogenationthereof; a titanium fiber sintered plate obtained by compression moldingtitanium fibers to sinter the preform; and a titanium fiber sinteredplate on a surface of which a plasma sprayed layer of metallic titaniumis formed, the last of which is disclosed in JP A 1999 (H11)-302891.

A porous conductive plate made of a prior art titanium sintered compactdescribed above, however, has the following problems.

Though a titanium powder sintered compact has an advantage that it issmooth on surfaces thereof and gives no damage on an adjacent filmelectrode laminates, the sintered compact has a fatal restraint that itis poor in press moldability and easily broken; therefore, it cannot befabricated with a small thickness and a large area. On the other hand,though a titanium fiber sintered plate is good in moldability and can beproduced with a small thickness and a large area, it has acute angledprotrusions and depressions on surfaces thereof with large spacingsbetween fibers. Therefore, if titanium fiber sintered plates are broughtinto press contact with an adjacent film electrode laminates, there is ahigh risk to damage the film electrode laminates. Furthermore, thereremains a problem to increase a contact resistance between the titaniumfiber sintered plate and the film electrode laminates.

In contrast to the sintered compacts described above, a titanium fibersintered plate disclosed in JP A 1999 (H11)-302891 is a sintered platein which a plasma sprayed layer of metallic titanium is formed on asurface of the titanium fiber sintered plate to thereby cancel acuteangled protrusions and depressions, and large spacings between fibers,and can be said to be excellent in moldability thereof andcontactability between the sintered plate and the film electrodelaminates.

Since in addition to requirement of an extra cost due to plasmaspraying, a void ratio and surface profiles of a titanium fiber sinteredplate are different from those of a plasma sprayed titanium layer on thesurface of the plate, an electric resistance increases at a bondinginterface therebetween, leading to an electric resistance of a porousconductive plate higher than to be expected from an apparent void ratio.As a result, in a water electrolytic cell used at a high currentdensity, for example, in the range of from 1 to 3 A/cm², a large voltagedrop results. Needless to say that such a voltage drop is not at allallowed in a fuel cell.

Moreover, a large change in void ratio at a bonding interface leads to aworry to adversely influence on permeability of a gas and a liquid.

On the other hand, as an ink dispersion plate for a large ink jetprinter, there has been requested a porous plate of, for example, athickness of as thin as 2 mm or less and an area of as large as 200mm×100 mm or more. This porous plate requires a small variation in voidratio from a nature of this kind. As such an ink dispersion plate, therehas been used a sintered plate made of irregular shaped powder ofstainless steel.

As the recent trend, a demand has started to be generated on a porousplate more excellent in corrosion resistance than a sintered plate ofstainless steel powder, for which it is considered to use titaniumpowder more excellent in corrosion resistance than stainless steel.

Though titanium in greatly excellent in corrosion resistance and acidproofness as compared with stainless steel, it is extremely poor inmoldability to the contrary. Hence, a general fabrication method for atitanium sintered plate has been thought to be such thathydrogenation/dehydrogenation titanium powder, which has been acceptedto be comparatively good in moldability, is molded with a die press,followed by sintering the preform and furthermore, another fabricationmethod is also described in JP A 1995 (H7)-238302 in which there is usedtitanium sponge powder, which is comparatively good in moldability,similar to the case of hydrogenation/dehydrogenation titanium powder.

Moreover, a different method is described in JP A 1996 (H8)-170107 inwhich a metal powder sintered plate uniform in void ratio is fabricatedby HIP.

The present inventors tried a procedure in whichhydrogenation/dehydrogenation titanium powder or titanium sponge powderis molded with a die press to sinter the preform for the purpose tofabricate a dispersion plate uniform in void ratio with a thickness asthin as 2 mm or less and an area as large as 200 mm×100 mm or more, andsince the dispersion plate was excessively thin, it was broken afterpress, having disabled fabrication thereof.

The present inventors tried fabrication of the dispersion platedescribed above by HIP only to find difficulty. The reason for thedifficulty is that a porous plate after sintering was not able to beseparated from a capsule maintaining a shape of a sintered compactduring HIP. Moreover, it is also difficult to select a material of whichthe capsule is made, which together with the above reason, causes afabrication cost to be raised to a very high value.

The present invention has been made in light of such circumstances andit is a first object to provide a titanium powder sintered compactexcellent in corrosion resistance, having a small maximum pore diameter,and showing a performance of a small pressure drop during usage as asintered titanium filter.

It is a second object of the present invention to provide a titaniumpowder sintered compact excellent in bending.

It is a third subject of the present invention to provide a cylindricalporous compact low in fabrication cost despite using titanium powder,and excellent in reverse-washing reproducibility while being used as apowder sintered filter.

It is a fourth object of the present invention to provide a metalsintered filter excellent in corrosion resistance and reverse-washingreproducibility.

It is a fifth object of the present invention to provide a porousconductive plate excellent not only naturally in moldability, but alsoin surface smoothness even without coating like plasma spraying, and inaddition, easy in production and excellent in economy.

It is a sixth object of the present invention to provide a highlycorrosion resistant porous plate capable of economically satisfying acondition to realize a uniform void ratio and a small thickness asrequired by an ink dispersion plate for use in a large ink jet printer.

DISCLOSURE OF THE INVENTION

A prior art titanium powder sintered filter was fabricated withhydrogenation/dehydrogenation titanium powder or titanium sponge powder.This is mainly because particles included in powder are in irregularshapes; therefore the powder is excellent in press moldability. In acase where particles are irregular in the shapes, a cavity diameter isvaried only if a mold is filled with powder; therefore, a necessityarises for making a cavity diameter uniform by press molding, which alsomakes press molding indispensable.

Such a titanium powder sintered compact is, however, very poor inbendability as described above. Furthermore, sincehydrogenation/dehydrogenation titanium powder or titanium sponge powderis composed of particles in irregular shapes, a cavity diameter is madeuniform at a comparatively small level by press molding, thereby causingpress moldability to be comparatively good. It is difficult to mold thepowder into a cylinder with a large height, however, and reverse-washingreproducibility is also poor when the titanium sintered compact is usedas a filter. Even with press molding applied, uniformity of a cavitydiameter is still insufficient and a skin layer receiving no pressmolding is conspicuously non-uniform in cavity diameter as describedabove.

In order to solve these problems, the present inventors focusedattention on spherical gas atomized titanium powder. Spherical gasatomized titanium powder is powder of titanium or a titanium alloyproduced by means of a gas atomization method and individual particlesare of a sphere with a smooth surface since the individual particles areformed by solidification during the time when melt spray of titanium ora titanium alloy is flying. Furthermore, particle diameters can be verymuch reduced down to as small as 100 μm or less on average and screeningis easily applied for classification by particle diameter.

Such spherical gas atomized titanium powder is excellent in fluidity andhas a good contactability between particles; therefore, a uniform andsufficient packing density can be attained in the powder filling asintering vessel without applying a pressure thereto. Then, by sinteringthe powder in the vessel, a porous compact with a high mechanicalstrength is fabricated without press molding and in thus fabricatedporous compact, adjacent spherical particles are fused to each other atcontact points and the fused points are uniformly distributed in thebulk thereof; therefore, it was found that in a case where a porouscompact was comparatively small in thickness, excellency in bendingcharacteristic was assured. Furthermore, a sintered compact in any shapeand any size including a cylinder is fabricated without press moldingand thus fabricated sintered compact has not only a sufficient strengthbut a uniform cavity therein with certainty and furthermore, a shape ofeach cavity is of a smooth spherical surface. In addition, by changingdiameters of particles in raw material powder, that is by adjustingdiameters of particles in use, diameters of cavities are controlled in awide range with a constant void ratio. A void ratio in a porous compactthus obtained is in the range of from 35 to 55% without applying apressure to powder in a sintering vessel.

A titanium powder sintered compact of the present invention has reachedits completion based on findings described above, and is characterizedby that the sintered compact is a plate-like porous compact obtained bysintering spherical gas atomized titanium powder and a void ratio of theporous compact is in the range of from 35 to 55%.

In a sintered titanium filter made of the titanium sintered compact, amaximum pore diameter can be controlled in the range of from 3 to 70 μmand a pressure drop can be restricted to a small value. Note that a voidratio of the porous compact, though detailed later, is not limited inthe range of from 35 to 55% in actual fabrication thereof. This isbecause a void ratio in the range of from 35 to 55% is especially suitedfor a variety of applications and, at the same time, obtained with easein fabrication thereof.

A titanium powder sintered compact of the present invention can acquireso excellent a bending characteristic that the sintered compact can bebent into a cylinder by restricting a thickness of the porous compact to500 μm or less. If a thickness of the porous compact is larger than 500μm, bending at room temperature is impossible. In a case where powderwith particles in irregular shapes such as hydrogenation/dehydrogenationtitanium powder, titanium sponge powder or the like is used instead ofspherical gas atomized titanium powder, uniformity of a cavity diametercannot be achieved in molding without applying a pressure to the powdereven if the plate thickness is 500 μm or less. What's worse, since fusedpoints between particles are distributed in non-uniformity, therelocally arise portions with shortage of strength, thereby disablingbending at room temperature.

The plate thickness is especially preferably 100 μm or less from theviewpoint of bendability at room temperature. It is preferable that thethinner the lower limit of the plate thickness is, the better it is fromthe viewpoint of bendability at room temperature, while in a case wherea ratio of a particle diameter/a plate thickness is excessively large,for example in a mono-particulate layer, a void ratio is larger than therange of from 35 to 55%, which is preferably applied to a metal powdersintered compact. Therefore, the plate thickness is preferably threetimes an average diameter of particles in powder in use.

While a shape of a titanium powder sintered compact is basically a flatplate, the sintered compact may assume one of other shapes, for examplea curved plate or the like and it is naturally possible to bend a flatplate into a plate with a semicircular shape in section, or a U letterin section, to work a flat plate into a corrugated plate, or to bend aflat plate into a cylinder according to a kind of application, withoutimposing any limitation on a particular shape in a molding stage or ausage stage.

A cylindrical porous compact of the present invention is a titaniumpowder sintered compact described above and formed by sinteringspherical gas atomized powder directly into a cylinder, and for example,a cylindrical titanium powder sintered filter can be provided as aproduct large in height and good reverse-washing reproducibility andfabricated at a low cost without using a press.

A metal sintered filter of the present invention is of a titanium powdersintered compact, in a titanium porous structure of which, a cavitydiameter is stepwise increased from one surface thereof to the othersurface, and which not only is excellent in reverse-washingreproducibility, but can also have a uniform void ratio independently ofincrease in cavity diameter.

That is, particle diameters of spherical gas atomized titanium powderaffect cavity diameters. By increasing, stepwise, a particle diameter ofused powder from one surface of a filter to the other surface, a cavitydiameter can be stepwise increased, thus enabling a layered structure inwhich plural porous layers are stacked in ascending order of cavitydiameter increasing stepwise. Since even in a case where particlediameters of spherical powder changes to another ones, a void ratio in asintered compact is fundamentally constant without a pressure applied tothe powder in a sintering vessel, cavity diameters can be changedwithout changing a void ratio. If a sintering temperature is changed,contact areas between particles also become different, therebycontrolling cavity diameters and in turn, controlling a void ratio aswell.

FIGS. 1(a) and 1(b) are image views showing a difference in structurebetween a prior art example metal sintered filter and an example metalsintered filter of the present invention, respectively.

In the prior art example shown in FIG. 1(a), there is used powderincluding irregular shaped titanium particles 1 such ashydrogenation/dehydrogenation titanium powder, titanium sponge powder orthe like as titanium powder, wherein finer particles are used in a layercloser to the surface thereof and a cavity diameter is smaller in alayer closer to the surface thereof. In this case, press moldability iscomparatively good and a void ratio is made uniform by press molding,while a degree of the uniformity in void ratio is insufficient. Sinceshapes of cavities 2 formed between titanium particles 1 are not made ofsmooth curved surfaces, it is difficult to remove solid matter.

In contrast to this, in the example of the present invention shown inFIG. 1(b), there is used powder including spherical titanium particles 1produced by means of a gas atomization method as titanium powder,wherein finer particles are used in a layer closer to the surfacethereof and a cavity diameter is smaller in a layer closer to thesurface thereof, whereas avoid ratio is constant even without applying apressure. Since shapes of cavities 2 formed between titanium particles 1are made of smooth curved surfaces each of a spherical surface, it iseasy to remove solid matter from the cavities 2.

The present inventors, furthermore, fabricated, on a trial base,sintered plates imagined as a power feeder in a polymer electrolytemembrane type water electrolyzer or a current collector in a solidpolymer fuel cell using spherical gas atomized titanium powder andfeatures, characteristics thereof and the like were evaluated. As aresult, the following facts were made clear.

Spherical gas atomized titanium powder is excellent in fluidity and thepowder in a sintering vessel fills the vessel at a sufficient densityeven without applying a pressure. If sintering the powder, (1) asufficient mechanical strength is ensured in a case of a shape even witha thin and large area, (2) void ratios preferable as a power feeder or acurrent collector can be obtained with simplicity without any specialoperation applied, and (3) a surface is high in smoothness and no feararises of being brought into close contact with an adjacent filmelectrode laminates to damage it even without coating by plasma sprayingor the like. Therefore, there are avoided a voltage drop due to increasein resistance at a bonding interface and an adverse influence onpermeabilities of a gas and a liquid.

That is, a sintered compact using spherical gas atomized titanium powderis not applied even with a pressure in the course of fabrication norapplied with surface coating after the fabrication, but shows a veryexcellent aptitude shown in terms of both performance and economy as apower feeder in a polymer electrolyte membrane type water electrolyzeror a current collector in a solid polymer fuel cell.

A porous conductive plate of the present invention has been developedbased on such findings and is a titanium powder sintered compact andused as a power feeder in a polymer electrolyte membrane type waterelectrolyzer or a current collector in a solid polymer fuel cell.

In contrast, with a prior art titanium sintered plate, it was difficultto fabricate a thin and large area sintered plate as requested inapplication to an ink jet dispersion plate for use in an ink jet printeras described above. Moreover, the present inventors conducted a trial inwhich hydrogenation/dehydrogenation titanium powder was not applied withdie pressing prior to sintering and sintered without a pressure appliedthereto, but with the result of obtaining no uniformity in void ratiorequired in a dispersion plate.

In order to solve this problem, the present inventors focused attentionagain on spherical gas atomized titanium powder. Since spherical gasatomized titanium powder is very excellent in fluidity and good incontactability between particles, a uniform and sufficient packingdensity can be attained by filling a sintering vessel with the powderwithout a pressure applied to the powder in the sintering vessel. Bysintering the powder in the sintering vessel, a porous compact in theshape of a thin plate with a high mechanical strength was fabricatedwithout press molding and in addition, in thus fabricated thin plate,adjacent spherical particles were fused to each other in point contactsand the fused points were distributed uniformly; therefore, it was foundthat a variation in void ratio in a plate surface was also small.

A highly corrosion resistant porous plate of the present invention hasbeen completed based on such findings and made of a titanium powdersintered compact described above and a ratio T/S of a plate thickness T(in mm) of the porous compact to an area S of the porous compact (inmm²) is controlled to be 1/10000 or less.

If the ratio T/S is in excess of 1/10000, a porous plate excellent inuniformity in void ratio can be fabricated by HIP but without usingspherical gas atomized titanium powder. A production cost thereof ishighly expensive, however. A highly corrosion resistant porous plate ofthe present invention is significant because of being provided at a verylow cost with uniformity in void ratio, but without using even pressmolding, let alone HIP naturally not used.

A variation in void ratio in a surface of the plate is preferably 3% orless in standard deviation. A porous plate with the uniformity inferiorthereto can be fabricated by a combination of powder including particlesin irregular shapes and press molding. The lower limit is especially notdefined since a smaller variation in void ratio is better. In thepresent invention, the variation can be 3% or less and can also be 1% orless.

As spherical gas atomized titanium powder used in a titanium powdersintered compact of the present invention, three kinds, for example,classified by a range of particle diameters are sold on the market. Thatis, the three kinds include fine particles of 45 μm or less in diameter,coarse particles in the range of from 45 to 150 μm in diameter andcoarser particles 150 μm or more in diameter, and the fine particles hasan average particle diameter of about 25 μm and the coarse particles hasan average particle diameter of about 80 μm.

An average particle diameter of spherical gas atomized titanium power ispreferably selected in the range of 150 μm or less. If the averageparticle diameter exceeds 150 μm, spacings between fused points betweenparticles are excessively wide; therefore, a possibility of breakage inbending is high. A void ratio, for example, in a case where a titaniumpowder sintered compact of a plate thickness in the vicinity of 500 μmis larger than the range of from 35 to 55% preferably applied to a metalpowder sintered filter. A relationship between a plate thickness andfused points between particles are desirably such that two or more fusedpoints are present within a plate thickness range. The lower limit isnot specifically defined since there is a tendency that workability isimproved with decrease in particle diameter.

A plate thickness of a porous compact, that is a titanium powdersintered compact of the present invention is 500 μm or less from theviewpoint of bendability described above and it is preferable for thethickness to be 100 μm or less in consideration of bendability at roomtemperature.

A void ratio can be attained in the range of from 35 to 55% in a casewhere spherical gas atomized titanium powder sold on the market is usedeven without applying a pressure to the powder in filling it in a vesselor sintering. According to an investigation having been conducted by thepresent inventors, the void ratios in the range are preferable for usein a metal powder sintered filter.

Particle diameters of spherical gas atomized titanium powder for used ina cylindrical porous compact of the present invention is notspecifically limited to those in any particular diameter range and noproblem arises with a level of commercial powder of this kind, while itis difficult to produce extremely fine particles industrially with agood yield even according to a gas atomization method. In a case wherecoarse particles are used to fabricate a thin porous compact, a contactarea between particles in titanium powder relative to a thicknessthereof is smaller, so there arises a worry of shortage of strength.This is because in a case where coarse particles are used in fabricationof a thin porous compact, the number of contact points between particlesin titanium powder is small. On the other hand, if a contact areabetween particles in titanium powder is increased so as to supplement adecrease in the number of contact points to thereby improve thestrength, a void ratio cannot fall inside the range of from 35 to 55%.Therefore, particle diameters are preferably in the range of from 10 to150 μm on average.

A void ratio of a cylindrical porous compact can be attained in therange of from 35 to 55% using spherical gas atomized titanium powdersold on the market even without a pressure applied in filling andsintering. According to an investigation having been conducted by thepresent inventors, void ratios in the range are preferable for use in ametal powder sintered filter.

A void ratio can be strictly controllable by adjustment of a sinteringtemperature, selection of particle diameters, adjustment of a pressureand the like. In a general tendency, with a higher sinteringtemperature, a contact area between particles increases and a cavitydiameter decreases, resulting in reduction in void ratio. Likewise, as aparticle diameter becomes smaller, a sinterability is improved with thesame sintering temperature held and as a result, a cavity diameter getssmaller, leading to a tendency of decreasing a void ratio. If a pressureis applied in filling powder into a vessel and sintering of the powder,a void ratio is reduced.

A cavity diameter can be controlled, similarly to the case of a voidratio, by adjustment of a sintering temperature, selection of particlediameters and the like. In a cylindrical porous compact of the presentinvention, a cavity diameter is made uniform without applying a pressbecause of excellency in fluidity of spherical gas atomized titaniumpowder. With particle diameters made more uniform, uniformity in cavitydiameter is further promoted. Anyway, specifications of a product issubstantially determined by specifications of raw material powder, whichmakes fabrication of the cylindrical porous compact simple.

A shape and size of a porous cylinder are properly determined by a shapeand size of a product such as a filter to be fabricated and in a case ofnatural filling without pressing, a shape and size of a product isgoverned by an inner shape and size of a sintering vessel.

Note that in JP No. 2791737 described above, the use of spherical gasatomized powder is described, while the spherical powder is used not information of a base section in the shape of a cylinder but in formationof a fine powder layer coated on the surface of the base section,wherein the base section is fabricated by sintering powder includingparticles of irregular shapes into a cylinder by cold isostaticpressing.

Cavity diameters are important in a metal sintered filter of the presentinvention. Cavity diameters are preferably selected in the range of from3 to 70 μm. That is, while in a highly corrosion resistant metalsintered filter, cavity diameters are desirably 70 μm or less inconsideration of filterabilty, spherical gas atomized powder of anaverage particle diameter of 10 μm or less is required to be used inorder to obtain cavity diameters of 3 μm or less, leading to a highfabrication cost.

A void ratio in the range of from 35 to 55% can be attained by usingspherical gas atomized titanium powder sold oh the market withoutapplying a pressure to the powder in filling and sintering. According toan investigation having been conducted by the present inventors, thevoid ratios in the range are preferable in a metal powder sinteredfilter.

No specific limitation is placed on a range of particle diameters ofspherical gas atomized titanium powder, and the powder of this kind atan off the shelf level described above can be non-problematically used,whereas extremely fine powder is difficult in industrial production evenaccording to a gas atomization method with a good yield. On the otherhand, a shortage of strength is worried in a thin porous compact usingcoarse particles since a contact area of particles of titanium powder issmall relative to a thickness of the thin porous compact. Therefore,particle diameters are preferably selected in the range of from 10 to150 μm on average so as to adapt for required cavity diameters.

No specific limitation is placed on a range of particle diameters ofspherical gas atomized titanium powder for use in a porous conductiveplate of the present invention and the powder of this kind at an off theshelf level is non-problematically used, whereas extremely fine powderis difficult in industrial production even according to a gasatomization method with a good yield. On the other hand, a shortage ofstrength is worried in a thin porous compact using coarse particlessince a contact area of particles of titanium powder is small relativeto a thickness of the thin porous compact. Therefore, particle diametersare preferably selected in the range of from 10 to 150 μm on average.

The range of from 35 to 55% in void ratio of a porous conductive platecan be attained by using spherical gas atomized titanium powder sold onthe market without applying a pressure in filling and sintering.According to an investigation having been conducted by the presentinventors, the range of the void ratio is preferable in consideration ofelectrical and mechanical properties of a porous conductive plate madeof a titanium powder sintered compact. Note that adjustment of a voidratio so as to be 35% or less can also be realized by applying apressure in filling and sintering and selecting other sinteringconditions.

A void ratio can be controllable by adjustment of a sinteringtemperature, selection of particle diameters, adjustment of a pressureand the like. In a general tendency, with a higher sinteringtemperature, a contact area between particles increases, resulting inreduction in void ratio. Likewise, as a particle diameter becomessmaller, a contact area between particles increases, leading to atendency of decreasing a void ratio. If a pressure is applied in fillingand sintering, a void ratio decreases. As particle diameters are largerrelative to a thickness of a porous conductive plate, there arises atendency of increasing a void ratio. With combinations of parameters orconditions described above adopted, a void ratio is controlledarbitrarily in a comparatively wide range. Note that an increase anddecrease in void ratio to extremes become causes for degradation ofreception/supply efficiency of water and gas in a reaction and shortageof a strength of a porous conductive plate.

A size of a porous conductive plate is properly selected depending on asize of a power feeder or a current collector to be fabricated.

An average particle diameter D of spherical gas atomized titanium powderfor use in a highly corrosion resistant porous plate of the presentinvention is preferably 150 μm or less. If the average particle diameterexceeds 150 μm, cavity diameters grow large, so a dispersion effect isdifficult to be attained. No specific limitation is imposed on the lowerlimit of an average particle diameter D since the smaller an averageparticle diameter is, the better it is.

A thickness T of a porous plate is preferably 2 mm or less and morepreferably 1 mm or less in order to reduce a pressure drop.

A void ratio is preferably in the range of from 35 to 55%. This isbecause if a void ratio is less than 35%, a problem arises that adispersibility is degraded and a pressure drop is increased. The upperlimit is reasonably 55% in consideration of geometry in a case wherespherical particles are used as powder.

A highly corrosion resistant porous plate of the present invention isespecially preferable as an ink dispersion plate for an ink jet printersmall in thickness and large in area, requiring a uniform void ratio anda high corrosion resistance, and greatly contributes to reduction infabrication cost for the dispersion plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) are image views showing a difference in structurebetween a prior art example metal sintered filter and an example metalsintered filter of the present invention, respectively.

FIG. 2 is an electron microscopic photograph of a titanium sinteredfilter obtained by sintering without applying a pressure using sphericalpowder particles produced by means of a gas atomization method fromtitanium sponge as a raw material in an embodiment of the presentinvention.

FIG. 3 is an electron microscopic photograph of a titanium sinteredfilter obtained by sintering without applying a pressure using particlesin irregular shapes obtained by pulverizing titanium sponge as a rawmaterial with a hydrogenation/dehydrogenation method

FIG. 4 is a graph showing, by comparison, relationships between a flowrate of a passing fluid and a pressure drop in Example 3 of the presentinvention and Comparative examples 4 to 6.

FIG. 5 is an image view of a titanium powder sintered compact showing asecond embodiment of the present invention.

FIG. 6 is a descriptive view for a fabrication method for a cylindricalporous compact showing a third embodiment of the present invention and asectional view showing a filling state of spherical gas atomizedtitanium powder.

FIG. 7 is a model sectional view of a metal sintered filter showing afourth embodiment of the present invention.

FIG. 8 is a descriptive view for a fabrication method for a porousconductive plate showing a fifth embodiment of the present invention anda sectional view showing an example filling state of spherical gasatomized powder.

FIG. 9 is a sectional view showing another example filling state ofspherical gas atomized powder.

FIG. 10 is a sectional view showing still another example filling stateof spherical gas atomized powder.

FIG. 11 is an image view of a highly corrosion resistant porous plateshowing a sixth embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Description will be given of embodiments of the present invention belowwith reference to the accompanying drawings.

Raw material powder of titanium or a titanium alloy used in embodimentsof the present invention is spherical particles of 150 μm or less(hereinafter referred to as spherical titanium powder for short)produced by means of a gas atomization method from titanium sponge.Since the spherical particles obtained by means of the gas atomizationmethod is powder solidified during flying of melt spray of titanium,surfaces of particles of particles included in the powder is extremelysmooth as compared with particles in irregular shapes in powder obtainedby pulverizing titanium sponge or in powder obtained byhydrogenation/dehydrogenation.

In a case where a filter is fabricated using the spherical titaniumpowder described above, diameters of particles of the powder aredesirably made uniform using a screen in order to achieve a desiredperformance. Then, a sintering vessel is filled with the sphericaltitanium powder uniform in particle diameter without applying a pressureto the powder therein. A void ratio of a sintering raw material fillingthe vessel without applying a pressure is adjustable in the range offrom 35 to 55% by adjusting a particle size distribution of thesintering raw material. By applying vibration to the spherical titaniumpowder prior to the sintering, a void ratio is reduced so as to fall inthe range of from 35 to 55%. There arises no chance of being 35% orless, however. Note that in a case where a pressure is applied infilling, a void ratio is reduced generally 35% or less.

While by sintering the spherical powder filling the sintering vesselwithout applying a pressure as described above, only contact pointsbetween spherical particles are fused to each other to bond, amechanical strength required by a filter is sufficiently ensured. Sinceby sintering the spherical powder at a temperature range much lower thanthe melting point of titanium, the spherical powder is sintered whilesustaining shapes of spherical particles prior to the sintering, a voidratio of the sintered compact does not change and the void ratio afterthe sintering remains in the range of from 35 to 55% unchanged from thatprior to the sintering. Note that as far as the sintering is performedat the low temperature range, a sintered compact can be obtained in therange of 35 to 55% in void ratio.

Since spherical titanium powder can be industrially produced so as to beas small as in the range of from 10 to 150 μm in average particlediameter by means of a gas atomization method, a spherical titaniumfilter can be fabricated with a maximum pore diameter in the range offrom 3 to 70 μm with the spherical titanium powder. That is, a titaniumfilter with fine pores and a small pressure drop can be fabricated witha high productivity. Note that if the spherical titanium powder fallsoutside and exceeds the range of from 10 to 150 μm in average particlediameter, it is impossible to obtain a sintered compact with a maximumpore diameter in the range of 3 to 70 μm.

On the other hand, while spherical powder can be produced according to arotating electrode method, an obtained average particle diameter ofspherical powder is generally 400 μm or more and it is difficult toindustrially produce spherical powder with an average particle diameterof 150 μm or less and therefore, much harder to industrially producespherical powder with an average particle diameter of 30 μm or less witha good yield.

A maximum pore diameter described above is measured with a mercuryporosimeter method. The mercury porosimeter method gets started withimmersing a specimen into mercury and then gradually raises a pressureof mercury. During the increase in pressure, as a pressure is raised,mercury intrudes into a pore with a smaller diameter; thereby obtaininga value discriminating between pore sizes of a porous compact. That is,a porous compact with a smaller maximum pore diameter has smaller poresand makes it possible to realize a filter so excellent in performance asto remove foreign matter with a smaller size.

In implementing the present invention, it is desirable to sinter rawmaterial spherical titanium powder filling a cylindrical vessel at atemperature in the range of from 650 to 1200° C. much lower than themelting point of titanium without applying a pressure in order toequally maintain a void ratio of the raw material spherical titaniumpowder in the sintered compact without reduction in the void ratio ofthe raw material spherical titanium powder in the course of thesintering. If a sintering temperature is lower than 650° C., sinteringis performed insufficiently, while if exceeding 1200° C., sinteredportions are not limited to contact points between particles, but thebodies of particles are molten together, with the result that originalshapes of spherical particles cannot be maintained to deform andcontract, decreasing a void ratio and in turn, increasing a pressuredrop.

Implementation of the present invention features no adoption of moldingwith a press which causes deformation of particles in powder; therefore,a sintered titanium filter can also be fabricated in a procedure inwhich a green preform is obtained by mixing spherical titanium powderwith a proper binder as in a doctor plate method or an extrusion methodand then, thus obtained green preform is degreased to remove the binderand vacuum sintered.

EXAMPLE 1

Billets were obtained from raw material titanium sponge and a meltthereof produced by electromagnetic induction heating was gas atomizedin an Ar gas atmosphere. Obtained titanium powder was classified byvibration screening to obtain spherical powder with an average particlediameter of 10 μm. A high density alumina vessel in the shape of asquare having one inner side of 100 mm and a depth of 3 mm was filledwith the powder without applying a pressure thereto and then the powderwas sintered keeping it at a vacuum degree of 7×10⁻³ Pa at 1000° C. for15 minutes without applying pressure to the powder.

EXAMPLE 2

A titanium sintered filter was fabricated in the same method andconditions as in Example 1 with the exception that the gas atomizedpowder was classified by vibration screening to obtain spherical powderwith an average particle diameter of 29 μm.

EXAMPLE 3

A titanium sintered filter was fabricated in the same method andconditions as in Example 1 with the exception that the gas atomizedpowder was classified by vibration screening to obtain spherical powderwith an average particle diameter of 124 μm. In FIG. 2, there is shownan electron microscopic photograph of the titanium sintered filter. Itis found that each of particles of the titanium sintered filter ismaintained in the shape of an unchanged sphere with many of voids.

EXAMPLE 4

A titanium sintered filter was fabricated in the same method andconditions as in Example 1 with the exception that the gas atomizedpowder was classified by vibration screening to obtain spherical powderwith an average particle diameter of 140 μm. Furthermore, the samevessel as in Example 1 was filled with the powder without applyingpressure, but followed by vibrations of 100 cycles imposed on the vesselwith a vibration machine. On this occasion, also different from Example1, the vessel was filled with the powder higher than 3 mm in heightprior to the vibrations so that 3 mm in height resulted after thevibrations.

EXAMPLE 5

A titanium sintered filter was fabricated in the same method andconditions as in Example 1 with the exception that the gas atomizedpowder was classified by vibration screening to obtain spherical powderwith an average particle diameter of 148 μm. Furthermore, the samevessel as in Example 1 was filled with the powder without applyingpressure, but followed by vibrations of 100 cycles imposed on the vesselwith a vibration machine. On this occasion, also different from Example1, the vessel was filled with the powder higher than 3 mm in heightprior to the vibrations so that 3 mm in height resulted after thevibrations.

In Examples 3, 4 and 5, an average particle diameter of the raw materialpowder was adjusted so that a maximum pore diameter of a sintered filterobtained in each of the examples was in the range of from 47 to 68 μm,wherein if a pressure is applied, the pressure can be adjusted for theadjustment. The reason why a maximum pore diameter was adjusted in therange of from 47 to 68 μm is that a maximum pore diameter of a sinteredfilter used in a gas chromatography apparatus satisfies a conditionrequiring the maximum pore diameter of 70 μm or less. Since a sinteredfilter with the same maximum pore diameter is desirably more excellentin corrosion resistance and smaller in pressure drop, filters in thesame shape were fabricated in Comparative examples 1, 2, and 4 to 6,described below, and pressure drops were compared with each other in acondition of a flow rate of 1 litter/min/cm².

COMPARATIVE EXAMPLE 1

Billets were obtained from raw material titanium sponge and a meltthereof produced by electromagnetic induction heating was gas atomizedin an Ar gas atmosphere. Obtained titanium powder was classified byvibration screening to obtain spherical powder with an average particlediameter of 212 μm. A high density graphite vessel in the shape of asquare having one inner side of 100 mm was filled with the powder andthen the powder was sintered keeping it at a vacuum degree of 7×10⁻³ Paat 1660° C. for 15 minutes under a pressure of 800 kg/cm² thereto toobtain a titanium sintered filter with a thickness of 3 mm.

COMPARATIVE EXAMPLE 2

A titanium sintered filter was fabricated in the same method andconditions as in Comparative example 1 with the exception that the gasatomized powder was classified by vibration screening to obtainspherical powder with an average particle diameter of 246 μm.Furthermore, a high density graphite vessel in the shape of a squarehaving one inner side of 100 mm was filled with the powder and then thepowder was sintered keeping it at a vacuum degree of 7×10⁻³ Pa at 1660°C. for 15 minutes under a pressure of 1200 kg/cm², which pressure wasdifferent from the temperature adopted in Comparative example 1, theretoto obtain a titanium sintered filter with a thickness of 3 mm.

COMPARATIVE EXAMPLE 3

A cylindrical titanium ingot is pulverized by means of a plasma rotatingelectrode method and the powder was classified by vibration screening toobtain spherical powder with an average diameter of 450 μm. A highdensity alumina vessel in the shape of a square having one inner side of100 mm and a depth of 3 mm was filled with the classified powder withoutapplying a pressure and then the classified powder was sintered keepingit at a vacuum degree of 7×10⁻³ Pa at 1000° C. for 15 minutes withoutapplying a pressure thereto to obtain a titanium sintered filter.

COMPARATIVE EXAMPLE 4

Stainless steel powder sold on the market produced by means of a wateratomization method was classified by vibration screening to obtainpowder including particles in irregular shapes with an average particlediameter of 147 μm. The classified powder was sintered in the samecondition as in Comparative example 3 to obtain a titanium sinteredfilter.

COMPARATIVE EXAMPLE 5

Powder obtained by pulverizing titanium sponge with ahydrogenation/dehydrogenation method was classified by vibrationscreening to obtain powder including particles in irregular shapes withan average particle diameter of 102 μm. The classified powder wassintered in the same condition as in Comparative example 3 to obtain atitanium sintered filter. In FIG. 3, there is shown an electronmicroscopic photograph of the titanium sintered filter. The sinteredcompact is composed of particles of irregular shapes.

COMPARATIVE EXAMPLE 6

Powder obtained by pulverizing titanium sponge with a mechanicalcrushing method was classified by vibration screening to obtain powderincluding particles in irregular shapes with an average particlediameter of 103 μm. The classified powder was sintered in the samecondition as in Comparative example 3 to obtain a titanium sinteredfilter.

Physical parameters of raw material powder used in Examples 1 to 5 andComparative examples 1 to 6 were compared and shown in table 1.Furthermore, in Table 2, there are shown physical parameters (voidratios, maximum pore diameters, particle diameters and pressure drops)of thus obtained sintered filters. Note that there are shown measuredparticle diameters of sintered filters only in Examples 1 to 4 of thepresent invention and Comparative example 3, where shapes of sphericalparticles in raw material powder are maintained after the sintering.Furthermore, pressure drops of a fluid are shown at a flow rate of 1litter/min/cm² by comparison. TABLE 1 Raw material powder Raw AverageMaterial Production Diameter of Powder Method Shape (μm) Example 1Titanium gas atomization Spherical 10 sponge method Example 2 Titaniumgas atomization ″ 29 sponge method Example 3 Titanium gas atomization ″124 sponge method Example 4 Titanium gas atomization ″ 140 sponge methodExample 5 Titanium gas atomization ″ 148 sponge method ComparativeTitanium gas atomization ″ 212 Example 1 sponge method ComparativeTitanium gas atomization ″ 246 Example 2 sponge method ComparativeTitanium plasma rotating ″ 450 Example 3 ingot electrode methodComparative Stainless Water Irregular 147 Example 4 steel atomizationingot method Comparative Titanium Hydrogenation/ ″ 102 Example 5 spongedehydrogenation method Comparative Titanium Mechanical ″ 103 example 6sponge crushing method

TABLE 2 sintered filter maximum pore Particle Pressure Sintering voidratio diameter diameter drop method (%) (μm) (μm) (kgf/cm²) Example 1Without 41 3  10 — pressure Example 2 Without 42 12  32 — pressureExample 3 Without 44 47 156 0.12 pressure Example 4 Without 37 48 1900.16 pressure Example 5 Without 44 68 146 0.11 pressure ComparativeUnder 34 49 — 0.42 Example 1 pressure Comparative Under 30 48 — 1.2 Example 2 pressure Comparative Without 47 140 460 — Example 3 pressureComparative Without 51 48 — 0.55 Example 4 pressure Comparative Without56 47 — 0.40 Example 5 pressure Comparative Without 61 49 — 0.38 example6 pressure

Note that an average particle diameter of spherical particles includedin a titanium sintered filter is measured in the following way. Adiagonal is drawn between opposed vertices in a field of view in theshape of a rectangle when observing with a microscope and measurement isperformed on diameters of all of selected particles, 50% or more of thecontour of each of which is viewed, among particles on the diagonal.Then, first 10 measured values are selected in descending order of adiameter to calculate an average thereof. The measurement is repeated 10times at different sites and 10 calculated average values are furtheraveraged to eventually obtain an average particle diameter of thespherical particles. It is found from Tables 1 and 2 that an averagespherical particle diameter of a titanium sintered filter obtainedaccording to this procedure is almost the same as that of correspondingraw material powder.

While in the above examples, titanium sponge was raw material, there canbe used as raw material: titanium scrap and titanium ingot. Furthermore,in a case where a sintered filter of a titanium alloy is fabricated, adesired titanium alloy ingot is used for producing raw material powder.

In Examples 3 and 4 and Comparative examples 1, 2, 4, 5 and 6 describedabove shown in Tables 1 and 2, particle diameters of raw material and asintering pressure are adjusted and sintered so that in each example, amaximum pore diameter of a sintered filter took 48±1 μm. It is foundfrom the results of the comparison tests that there is a greatdifference in pressure drop between Examples 2 and 3 in which sinteringwas performed without applying a pressure using powder with an averageparticle diameter of 181 μm or less, and Comparative examples 1 and 2 inwhich sintering was performed under a pressure using powder with anaverage particle diameter of 200 μm or more, though the powder producedaccording to the same gas atomization method was used as raw material inboth example groups, and it is further found that a sintered filterfabricated by execution of the present invention has a smaller pressuredrop.

It is found that a pressure drop is large in any of sintered filters ofComparative examples 4 to 6 fabricated by sintering powder includingparticles in irregular shapes produced according to a water atomizationmethod, a hydrogenation/dehydrogenation method, a mechanical crushingmethod except for a gas atomization method without applying a pressure.What's worse, the stainless steel sintered filter of Comparative example4 has a problem of poor corrosion resistance. Note that in FIG. 4, thereare shown relationships between a flow rate of a passing fluid and afluid pressure drop in Example 3 and Comparative examples 4, 5, and 6.While in any case, a pressure drop is larger with an increased flowrate, a pressure drop in Example 3 of the present invention is thesmallest.

FIG. 5 is a model sectional view of a titanium powder sintered compactshowing a second embodiment of the present invention.

A sintering vessel in the shape of a dish made of high density aluminais filled with spherical gas atomized titanium powder 11 having aprescribed average diameter and thereafter, the spherical gas atomizedtitanium powder 11 is vacuum sintered without applying a pressure,thereby fabricating a porous sintered compact 10 in the form of a thinplate.

A plate thickness T of the sintered compact 10 is 500 μm or less.Adjacent spherical particles are fused to each other in point contactand the sintered compact 10 of a plate thickness of 500 μm or lessexerts an excellent bending characteristic. That is, portions whereadjacent particles are fused to each other in point contact areuniformly distributed throughout all of a titanium powder sinteredcompact using spherical gas atomized powder to thereby cause no localconcentration of a bending stress, leading to excellency in a bendingcharacteristic of the sintered compact.

A sintering temperature is preferably selected in the range of from 650to 1200° C. much lower than the melting point of titanium. If asintering temperature is lower than 650° C., sintering is notsufficiently performed. If exceeding 1200° C., sintering portions arenot limited to contact points between individual particles but thebodies of particles are molten together, so there arises a risk that avoid ratio and cavity diameters at proper levels cannot be ensured. Asintering temperature is changed in the range of temperatures, therebycontrolling a void ratio and cavity diameters. Furthermore, a bendingcharacteristic is also controlled.

As examples of the present invention and comparative examples thereof,there were fabricated titanium powder sintered compacts each in theshape of a thin plate with kinds of plate thickness and commonly in theshape of a square having one side of 150 mm using spherical gas atomizedtitanium powder sold on the market described above, that is fineparticles in the range of 45 μm or less (an average particle diameter of25 μm) and coarse particles in the range of from 45 to 150 μm (anaverage particle diameter of 80 μm).

Furthermore, a titanium powder sintered compact in the shape of a thinplate with similar dimensions was fabricated, as a prior art example, bya press in molding using hydrogenation/dehydrogenation titanium powder(an average particle diameter of 25 μm) sold on the market.

Breakage states of thus fabricated titanium powder sintered compact inthe shape of a thin plate wound around a cylinder with an outer diameterof 40 mm (a radius of curvature of 20 mm) were investigated to therebycompare between bending characteristics. Results are shown in Table 3.TABLE 3 average particle presence or Thickness T diameter D absence ofμm μm Void ratio % breakage Example 1 100 80 67 ◯ Example 2 400 80 50 ◯Example 3 500 80 47 ◯ Comparative 600 80 44 X Example 1 Example 4 100 2555 ◯ Example 5 400 25 48 ◯ Example 6 500 25 45 ◯ Comparative 600 25 42 XExample 2 Prior Art 100 25 62 X Example 1 Prior Art 400 25 56 X Example2 Prior Art 500 25 53 X Example 3 Prior Art 600 25 51 X Example 4

As understood from Table 3, in a case where spherical gas atomizedtitanium powder was used as titanium powder and a plate thickness is 500μm or less, an excellent characteristic can be attained independently ofparticle diameters in the plate (in a case of either fine particles orcoarse particles).

FIG. 6 is a descriptive view for a fabrication method for a cylindricalporous compact showing a third embodiment of the present invention and asectional view showing a filling state of spherical gas atomizedtitanium powder.

A sintering vessel 20 made of high density alumina is constructed of: aninner mold part 21 in the shape of a cylinder; an outer mold part 22 inthe shape of a cylinder arranged at the outside of the inner mold part21 with a prescribed clearance therebetween; a fixing mold part 23arranged at the outside of the outer mold part 22 to fix the outer moldpart 22; a spacer 24 in the shape of a ring arranged at the lowestposition of the sintering vessel 20 between the inner mold part 21 andthe outer mold part 22 in order to form an annular space 25therebetween.

The inner mold part 21 is divided into two pieces obliquely to theheight direction for removal and, together with the spacer 24 in theshape of a ring, inserted inside the outer mold part 22. The outer moldpart 22 is also divided into two pieces in a circumferential directionfor removal and firmly held by the fixing mold part 23 outside the outermold part 22 into a coalesced state.

The sintering vessel 20 is assembled to form a clearance 25 of anannular shape in section on and above the spacer 24 between the innermold part 21 and the outer mold part 22. The clearance 25 in an annularshape in section is filled with spherical gas atomized titanium powder30 without applying a pressure. Then, the spherical gas atomizedtitanium powder 30 in the sintering vessel 20 was vacuum sinteredwithout applying a pressure.

In such a way, a cylindrical titanium powder sintered filter isfabricated. In the filter, contact states between particles are good andsizes of cavities formed between particles are made uniform; therefore,a sufficient strength and uniform cavity diameters can be ensured. As aresult, a tall filter can be fabricated at a low cost. With decrease inparticle diameters, cavity diameters can be reduced without changing apacking density. By making particle diameters uniform, uniformity incavity diameters can be further improved. Furthermore, since each ofshapes of cavities is enclosed by a smooth curved surface, there is asmall chance for plugging pores and the filter is excellent inreverse-washing reproducibility.

A sintering temperature is preferably in the range of from 650 to 1200°C. much lower than the melting point of titanium. If a sinteringtemperature is lower than 650° C., sintering is not sufficientlyperformed. If exceeding 1200° C., sintering portions are not limited tocontact points between individual particles but the bodies of particlesare molten together, even without applying a pressure, so there arises arisk that a void ratio and cavity diameters at proper levels cannot beensured.

By changing a sintering temperature, a void ratio and cavity diametersare controlled as described above. Hence, an optimal sinteringtemperature differs according to particle diameters in the powder to besintered. For example, it is desirable that a sintering temperature isespecially in the range of from 850 to 1200° C. for coarse particles inthe range of from 45 to 150 μm in particle diameter. If a sinteringtemperature is lower than 850° C., there is a risk that sintering is notsufficiently performed. On the other hand, in a case where fineparticles of 45 μm or less are used, it is desirable that a sinteringtemperature is especially in the range of 650 to 850° C. sincesufficient sinterability is ensured even in a comparatively lowtemperature range.

As an example of the present invention, fabricated in the methoddescribed above was a cylindrical titanium sintered filter with a heightof 250 mm, an outer diameter of 60 mm, an inner diameter of 56 mm and awall thickness of 2 mm. Used spherical gas atomized titanium powder wasin the range of from 45 to 150 μm in particle diameter, an atmosphere inthe sintering furnace was in a vacuum state, a sintering temperature was1100° C. and a sintering time was 30 minutes. Investigation wasperformed about a void ratio and cavity diameters of a fabricated filterat 5 points in the height direction.

As a comparative example, a titanium sintered filter in the shape of thesame cylinder was fabricated using hydrogenation/dehydrogenationtitanium powder sold on the market (in the range of from 45 to 150 μm inparticle diameter) while additionally using press molding. Investigationwas performed about a void ratio and cavity diameters of a fabricatedfilter at 5 points in the height direction.

Results of the investigations are shown in Tables 4 and 5. In theexample, the void ratios and cavity diameters are made uniform in theheight direction despite no use of a press, whereas in the comparativeexample, filling of titanium powder was not uniformly performed despiteapplication of press molding; therefore, variations were large in voidratios and cavity diameters. TABLE 4 measuring Void ratio (%) points 1 23 4 5 averages Example 41 42 42 42 43 42 Comparative 63 57 56 65 54 59Example

TABLE 5 measuring Average Cavity Diameter (μm) points 1 2 3 4 5 averagesExample 26 27 26 25 23 25 Comparative 41 36 41 44 33 39 Example

Comparison was performed between both cases in reverse-washingreproducibility. That is, a solution obtained by mixing silica beadshaving an average diameter of 10 μm into water at a concentration of 10mg/litter was filtered through filters so that an increase in weight ofa filter after drying is constant, thereafter, reverse-washing wasperformed on the filters for a prescribed time at an air pressure of 5kgf/cm² and after drying, the filters were weighed to obtain a changebetween weights before and after the procedure described above, therebyhaving evaluated reverse-washing reproducibility. In the example, 94% ofthe increase in weight was removed by reverse-washing, while in thecomparative example, only 78% were removed.

Note that while in the embodiments described above, cylindrical productswere fabricated directly form the powder, it is possible that two partsof the product in the shape of a semi-cylinder are separately fabricatedand thereafter, the two parts are welded to complete a cylinder.Besides, a shape of a sintered product is not limited to a cylinder, butmay be a long, hollow body with a straight side and a section in theshape of a polygon or the like.

FIG. 7 is a model sectional view of a metal sintered filter showing afourth embodiment of the present invention.

A sintering vessel made of high density alumina is filled with sphericalgas atomized titanium powder with a prescribed average particle diameterwithout applying a pressure and thereafter, the spherical gas atomizedtitanium powder is vacuum sintered without applying a pressure, therebyfabricating a first plate-like porous compact 41.

A second plate-like porous compact 42 is fabricated in a similar waywith the exception that spherical gas atomized titanium powder with anaverage particle diameter larger than the spherical gas atomizedtitanium powder used in the first plate-like porous compact 41. In thisprocess, a sintering temperature is adjusted so that the same void ratioas in the first plate-like porous compact 41 is realized.

A third plate-like porous compact 43 is fabricated in a similar way withthe exception that spherical gas atomized titanium powder with anaverage diameter larger than the spherical gas atomized titanium powderused in the second plate-like porous compact 42. In this process, asintering temperature is adjusted so that the same void ratio as in thefirst plate-like porous compact 41 and the second plate-like porouscompact 42 is realized.

The fabricated three plate-like porous compacts 41, 42 and 43 aresuperposed one on another and sintered to thereby fabricate a sinteredfilter 40 of a three-layer structure. Since the fabricated sinteredfilter 40 is constructed with three kinds of powder each different indiameters of particles used therein from the other, cavity diameters ina porous compact are increased in order of the plate-like porouscompacts 41, 42 and 43. A void ratio is almost constant in plate-likeporous compacts because of filling without applying a pressure. Avariation among cavity diameters in each porous compact is small andshapes thereof each are enclosed with a smooth curved surface anduniform.

A performance of the sintered filter 40 as a filter is excellent byadopting a design in which a treated liquid is caused to pass throughthe plate-like porous compacts 41, 42 and 43 in the order to therebytrap almost all of solid matter in the treated liquid with theplate-like porous compact 41 having cavity diameters smallest in adiameter range, thereby obtaining excellent reverse-washingreproducibility. That is, since in this design the solid matter is notfiltered out on the plate-like porous compacts 42 and 43 in adistributed state and in addition, shapes of cavities in the plate-likeporous compact 41 are smooth and uniform, the solid matter trapped inthe cavities is removed smoothly in reverse-washing.

Sintering temperatures in respective sintering are preferably selectedin the range of from 650 to 1200° C. much lower than the melting pointof titanium. If a sintering temperature is lower than 650° C., sinteringis not sufficiently performed. If exceeding 1200° C., sintering portionsare not limited to contact points between individual particles but thebodies of particles are molten together even without applying apressure, so there arises a risk that void ratios and cavity diametersat proper levels cannot be ensured. By changing sintering temperature inthis range, void ratios and cavity diameters are controlled as describedabove.

As an example of the present invention, a titanium sintered filter of athree-layer structure was fabricated according to the method describedabove. A thickness of each layer was 1 mm, (3 mm in total). Averagediameters of particles included in spherical gas atomized titaniumpowder used in layers were 20 μm, 60 μm and 100 μm, respectively, andmaximum cavity diameters of the layers were 6 μm, 22 μm and 37 μm,respectively. The void ratios of the layers were all 45%.

As a comparative example, a titanium sintered filter of a similarstructure was fabricated using hydrogenation/dehydrogenation titaniumpowder sold on the market. In fabrication of three plate-like porouscompacts, a press was necessary for molding and making cavity diametersuniform. Variations were observed in void ratios of respective layersand 55%, 48% and 37%.

Comparison was performed between both cases in reverse-washingreproducibility. That is, a solution obtained by mixing silica beadshaving an average diameter of 10 μm into water at a concentration of 10mg/litter was filtered through the filters so that an increase in weightof a filter after drying is constant, thereafter, reverse-washing wasperformed on the filters for a prescribed time at an air pressure of 5kgf/cm² and after drying, the filters were weighed to obtain a changebetween weights before and after the procedure described above, therebyhaving evaluated reverse-washing reproducibility. In the example, 97% ofthe increase in weight was removed by reverse-washing, while in thecomparative example, only 83% was removed.

In the embodiment described above, the plate-like porous compacts eachdifferent in cavity diameters therein from another were individuallyfabricated in advance, while a similar layered structure can also beobtained in a procedure in which titanium particle layers each differentin diameters of particles therein from another are sequentially stackedand sintered. Incidentally, a layered structure shown in FIG. 1(b) isfabricated by mans of the latter method.

FIGS. 8 to 10 are descriptive views of fabrication methods for porousconductive plates showing a fifth embodiment of the present inventionand sectional views showing filling states of particles included inspherical gas atomized powder.

As shown in FIG. 8, first of all, a sintering vessel 60 made of highdensity alumina is filled with spherical gas atomized titanium powder 50having prescribed particle diameters without applying a pressure. Ashape of an inner space of the sintering vessel 60 is of the shape of athin plate corresponding to a shape of a porous conductive plate to befabricated. Then, the spherical gas atomized titanium powder 50 fillingthe sintering vessel 60 is vacuum sintered without applying a pressure.

A sintering temperature is preferably selected in the range of from 650to 1200° C. much lower than the melting point of titanium. If asintering temperature is lower than 650° C., sintering is notsufficiently performed. If exceeding 1200° C., sintering portions arenot limited to contact points between individual particles but thebodies of particles are molten together even without applying apressure, so there arises a risk that a void ratio at a proper levelcannot be ensured.

By means of such a method, there were fabricated three kinds of porousconductive plates, commonly in the shape of a square having one side of50 mm each, and respective thickness of 1 mm, 0.5 mm and 0.2 mm as anexample of the present invention.

Spherical gas atomized titanium powder was powder sold on the market asdescribed above and powder of coarse particles (in the range of from 45to 150 μm) was used for fabricating the porous conductive plates of 1 mmand 0.5 mm in thickness, respectively, while powder of fine particles(of 45 μm or less) was used for fabricating the porous conductive plateof 0.2 mm in thickness. A degree of a vacuum was 7×10⁻³ Pa and asintering temperature was about 1000° C. for the coarse particles whilebeing about 800° C. for the fine particles. Furthermore, a temperatureholding time was a constant value of about 15 minutes for both of thecoarse particles and the fine particles. The void ratios of thefabricated porous conductive plates were all about 45%.

Electrical resistance of thus fabricated porous conductive plates weremeasured with a four-probe method with the results that the porousconductive plate of 1 mm in thickness had 10 mΩ, the porous conductiveplate of 0.5 mm in thickness had 15 mΩ and the porous conductive plateof 0.2 mm in thickness had 12 mΩ because of the use of the powder offine particles in this last case. As to physical conditions of onesurfaces of the plates, the one surfaces were planarized since particlesincluded in spherical gas atomized titanium powder were arranged inconformity with the upper surface profile of the bottom of the sinteringvessel. Since spherical gas atomized titanium powder is good influidity, a void ratio is comparatively uniform throughout all of aporous conductive plate.

For the purpose of comparison, hydrogenation/dehydrogenation titaniumpowder sold on the market (in the range of from 50 to 150 μm in particlediameter with an average particle diameter of 100 μm) was sintered tofabricate porous conductive plates that were each in the shape of asquare of 50 mm in one side, had thickness values of 1 mm and 0.5 mm,respectively, and commonly had a void ratio of 45%. Molding with a presswas necessary to attain a void ratio of 45%. Electrical resistancevalues were equal to those in the example, while strength values wereinsufficient. This is inferred because powder of particles in irregularshapes are used, therefore titanium particles are not uniformly bondedtherebetween. This non-uniformity in bonding between particles isresulted in variations in void ratios in all of a porous conductiveplate.

On the other hand, a titanium fiber sintered plate sold on the market(with a thickness of 0.8 mm) has a void ratio as large as 60% andelectrical resistance was as high as 30 mΩ. Though a strength wassufficient, on a surface thereof were so much of fine protrusions thatthe sintered plate cannot be brought into press contact with a filmelectrode laminates. Spherical gas atomized titanium powder sold on themarket described above was plasma sprayed on one surface of the titaniumfiber sintered compact to a thickness of 0.2 mm, totaling 1 mm as awhole. Though a void ratio of all of the sintered compact assumed 45%and the one surface was planarized, an electrical resistance was stillas large as 20 mΩ, which was twice as high as in the example.

In the example described above, while a sintering temperature in thecase where the coarse particles were used was about 1000° C., a voidratio of a porous conductive plate at a sintering temperature of 1100°C. was reduced to about 40%. A void ratio of a porous conductive platein the example at a sintering-temperature of 900° C. increased to about50%. Any of the porous conductive plates was high in strength, excellentin surface smoothness and low in resistance.

As a method for raising a surface smoothness to a higher level, there isexemplified a method in which a sintering vessel with a necessary sizeis filled with spherical gas atomized titanium powder while givingvibrations to the powder. With a vibration filling adopted, as shown inFIG. 9, a surface smoothness can be improved not only on a surface incontact with the upper surface of the bottom of the sintering vessel 60,but also on the surface of the opening side and in addition, a voidratio is made more uniform. As shown in FIG. 10, it is effective to usea sintering vessel 60 constructed so that a plate-shaped space formedinside the vessel is vertically long. With an inner plate like spaceextended vertically adopted, spherical gas atomized titanium powder 50filling the space receives loads in the plate thickness direction causedby a weight of itself to improve surface smoothness on both surfaces. Inany of the methods, increase in packing density accompanies reduction invoid ratio and both can be used in parallel.

As molding methods, there may be exemplified in addition to combinednatural filling and vacuum sintering: a doctor blade method, aninjection molding method, an extrusion method and the like, with any ofwhich it is allowed that a green preform is prepared using a mixture ofspherical gas atomized titanium powder with a binder, followed bysequentially removing a binder from the green preform and sintering.Furthermore, it is possible to roll a porous sintered conductive plateafter sintering, or alternatively, to roll a green preform, therebyenabling more of surface smoothness and adjustment of a void ratio inthe plate. Still furthermore, it is also effective for surfacesmoothness to narrow a range of a particle diameter distribution ofspherical gas atomized titanium powder.

FIG. 11 is a model sectional view of a highly corrosion resistant porousplate showing a sixth embodiment of the present invention.

A sintering vessel in the shape of a dish made of high density aluminais filled with spherical gas atomized titanium powder 71 having aprescribed average particle diameter and thereafter, the spherical gasatomized titanium powder 71 is vacuum sintered without applying apressure, thereby fabricating a thin, large area porous plate 70 with ahigh corrosion resistance.

Here, a plate thickness T of the porous plate 70 is 1/10000 times orless of a numerical value of the area S. That is, T/S≦1/10000. Sinceadjacent spherical particles are in point contact and fused to eachother and sizes of cavity 72 formed between particles are uniform,uniformity in void ratio in a porous plate is high and the uniformityincreases as particle diameters are made more uniform, satisfying arequirement for a standard deviation being 3% or less.

A sintering temperature is preferably selected in the range of from 650to 1200° C. much lower than the melting point of titanium. If asintering temperature is lower than 650° C., sintering is notsufficiently performed. If exceeding 1200° C., sintering portions arenot limited to contact points between individual particles but thebodies of particles are molten together even without applying apressure, so there arises a risk that void ratios and cavity diametersat proper levels cannot be ensured. A void ratio is controlled bychanging a sintering temperature within the temperature range.

As an example of the present invention, a porous plate in the shape of asquare with one side of 200 mm and having a thickness of 2 mm wasfabricated using the above described spherical gas atomized titaniumpowder sold on the market, that is fine particles in the range of 45 μmor less in particle diameter (with an average particle diameter of 25μm) and coarse particles in the range of from 45 to 150 μm in particlediameter (with an average particle diameter of 80 μm). It isT/S=1/20000. A high density alumina vessel was used as a sinteringvessel and the vessel was filled with the spherical gas atomizedtitanium powder without applying a pressure, followed by vacuumsintering without applying a pressure. Conditions for sintering were at800° C. for 1 hour or the case where the fine particles and at 1000° C.for 1 hour for the case where the coarse particles.

As Comparative example 1, hydrogenation/dehydrogenation powder sold onthe market (with an average particle diameter of 25 μm) was used andconditions for sintering were the same as in the example described abovewith the exception of a sintering temperature of 800° C.)

The void ratios were measured at 5 points (A to E) on a surface offabricated porous plate. The measuring points were 5 points on adiagonal drawn between opposed vertices of a square with one side of 200mm with which points together with both vertices the diagonal is dividedinto 6 segments equal in length to one another. The void ratios wereobtained in a procedure in which thickness values, areas and mass valueswere measured on five square samples, each in the shape of a square withone side of 20 mm, and having measuring points at respective centersthereof to thereby obtain apparent densities and to further calculatethe void ratios according to the following expression. In Table 6, thereare shown the void ratios at respective measuring points, average valuesand standard deviations thereof.Void ratio (%)=(1−an apparent density/a true density of titanium)×100TABLE 6 Void ratios (%) Standard A B C D E Averages deviations Example 142 41 43 40 41 41 1 Example 2 43 39 46 42 46 43 3 Comparative 41 53 3960 36 46 10 Example 1

As is understood from Table 6, spherical gas atomized titanium powder isused, thereby enabling fabrication of a porous plate even with athickness as thin as 2 mm or less in plate thickness and with uniformvoid ratios.

A similar shape was tried to obtain by means of HIP using the powderused in Comparative example 1. Since when HIP was used, a porous platewas not able to be separated from a capsule made of tantalum withoutbreaking the plate, it was impossible to fabricate a porous plate in theshape of a square with one side of 200 mm and a thickness of 2 mm. Theminimum thickness of a porous plate that can be fabricated by means ofHIP was a thickness of 5 mm in a porous plate in the shape of a squarewith one side of 200 mm (wherein T/S=1/8000).

While a sintered compact in a similar shape was tried to fabricate bysintering a preform after molding the powder used in Comparative example1 with a die press, the preform is excessively thin, so it is brokenafter pressing and the process was not able to enter even into asintering step. The minimum thickness that can be fabricated accordingto the die press is a thickness of 5 mm in a porous plate in the shapeof a square with one side of 200 mm (wherein T/S=1/8000).

INDUSTRIAL APPLICABILITY

As described above, a titanium powder sintered compact of the presentinvention can be provided as a titanium sintered filter with a maximumpore diameter of 70 μm or less, small in pressure drop and excellent infiltration performance while maintaining an average particle diameterand a void ratio of a preform of raw material spherical powder.

A titanium powder sintered compact of the present invention can beprovided with a high bending characteristic as far as spherical gasatomized titanium powder is used and a plate thickness is restricted to500 μm or less; therefore, there can be fabricated with the titaniumpowder sintered compact: for example, filter elements in athree-dimensional shape, such as a cylinder and a corrugated plate, adispersion element and the like at a low cost without using CIP.

A cylindrical porous compact even with a large height of the presentinvention can also be fabricated without applying a pressure anduniformity in quality in the height direction is excellent despite noapplication of a pressure. Therefore, high quality cylindrical titaniumpowder sintered filters with various sizes can be economicallyfabricated. Furthermore, excellent reverse-washing reproducibility canbe imparted to the filters.

Since a highly corrosion resistant metal sintered filter of the presentinvention is fabricated with titanium powder, the filter is veryexcellent in corrosion resistance. Furthermore, since in a highlycorrosion resistant metal sintered filter of the present invention, acavity diameter stepwise increases from one surface to the othersurface, sizes of cavities in each layers are uniform and shapes ofcavities each are formed with a smooth curved surface; therefore, thesintered filter is excellent in reverse-washing reproducibility. Inaddition, since a void ratio can be made constant between one surfaceand the other surface, an adverse influence on a liquid permeability canbe avoided. Since a fabrication process is simple, a fabrication costcan be restricted to a low value.

Moreover, since a porous conductive plate of the present invention ismade of a sintered compact of spherical gas atomized titanium powder,which makes itself excellent in moldability, a thin, large area productcan be fabricated with simplicity. Besides, since a surface smoothnessis excellent even without coating such as plasma spraying, aprotectability for and contactability with a thin film electrodelaminates can be improved without accompanying increase in electricalresistance, causing economy to be also excellent. With such advantages,there can be provided a powder feeder and a current collector, both witha high performance at a low cost.

Besides, a highly corrosion resistant porous plate of the presentinvention can be economically fabricated as a plate too thin to befabricated with HIP, even without pressing and, furthermore, uniformityin void ratio can be even higher than a product by means of pressing.Therefore, thin porous plates uniform in void ratio can be fabricated ata very low cost to therefore, economically provide a high qualityproduct applicable to, for example, an ink dispersion plate for use inan ink jet printer.

1-16. (canceled)
 17. A titanium powder sintered compact made ofplate-like porous compact, obtained by sintering spherical gas atomizedtitanium powder, having a void ratio of the porous compact in the rangeof from 35 to 55%, and a thickness of the porous compact of 500 μm orless.
 18. The titanium powder sintered compact according to claim 1,fabricated by filling without applying a pressure and sintering withoutapplying a pressure.
 19. The titanium powder sintered compact accordingto claim 1, wherein an average of particle diameters of the sphericalgas atomized titanium powder is in the range of from 10 to 150 μm.
 20. Asintered titanium filter made of the titanium powder sintered compactaccording to claim
 1. 21. The sintered titanium filter according toclaim 4, wherein the cavity diameter is in the range of from 3 to 70 μm.22. A cylindrical filter obtained by bending the titanium powdersintered compact according to claim 1.